Fragile X Syndrome (FXS) is the leading cause of inherited mental retardation, and has been characterized by specific deficits in visual processing abilities associated with the parietal lobe of the brain. Significant research efforts have been made toward understanding the molecular mechanisms and cognitive and behavioral consequences of FXS in older children and adults, but much of the early development of the disorder remains to be examined. The overarching goal of the current proposal is to further understand this visual deficit by disentangling the spatial from the temporal aspects of visual attention in infants with FXS, and comparing them to those observed in mental age-matched typically developing infants. This will be accomplished by psychophysically determining individual spatiotemporal contrast detection thresholds as a measure of low-level, pre-attentive processing, and thresholds for spatial crowding and flicker phase detection indicators of the spatial and the temporal visual attention limit, respectively. Differences in visual attention have been used as an early indicator of later general intellectual function and a predictor of developmental outcome, and thus, the knowledge gained from this research proposal will be vital for the design of behavioral interventions and education-based treatments specific to young children with FXS. Given our understanding of the role of FMRP in normal synaptic formation, maturation, and plasticity during development, and the literature documenting the implications of the first 3 years of life on brain development and behavioral trajectories, it is expected that the most influential interventions are those provided early in life during the period of rapid proliferation and pruning of neural connections. Additionally, with the likelihood that newborn screening for FXS will begin in the near future, there is an even greater urgency to study basic visual processes that lead to early deficits in cognitive functioning. With a greater number of infants diagnosed before the age of 12 months, early intervention design will rely on results from studies such as the one proposed here to inform the framework of such therapies and ultimately promote optimal long-term development in individuals with FXS. This research has future implications for children with FXS and society as a whole.